A microwave ferrite junction circulator is quite the versatile microwave device—it can be used as a circulator, isolator or a switch. The three-port microwave ferrite junction circulator, also referred to as a Y-junction circulator, is one of the most commonly used junction circulators. It is a three-port non-reciprocal device, which provides routing for forward signals and re-routing (with suppression) for reverse signals.
A typical single junction circulator/isolator includes a stripline conductor circuit, described as a centre conductor enveloped with a ferrite material which is biased by a magnetic system to provide the desired non-reciprocal operation. The circulation of the signals, clockwise or counter-clockwise, is achieved and controlled by the biasing magnetic field generated by the magnetic system.
An example of a single junction circulator is shown in FIG. 1a (PRIOR ART). The isolation of a circulator is directly correlated to (and usually the same as) the return loss of the isolated port. The conventional design of a stripline conductor circuit used in a three-port junction circulator has three ports and impedance matching provided by a quarter-wave transformer in each port to attain a typical return loss of 20 dB. When operating as an isolator, port 3 of the three-port junction circulator is connected to a resistive load to suppress the reverse signals re-routed from port 2—as seen in the exemplary embodiment of a single junction isolator of FIG. 1b (PRIOR ART). Hence, in known prior art devices (circulator or isolator), the stripline conductor circuit is usually designed to match the impedance of the port to that of the transmission line or resistive load.
Increasing demands for a higher level of isolation between the input and output signals in modern wireless systems and sub-systems severely limits the isolation performance achievable from a single junction circulator. In many systems used for high radio frequency (RF) power applications, for example in transmit modules for communication networks or equipment, the typical ˜20 dB level of isolation is not sufficient to provide the required isolation between forward and reflected or reverse signals. Electrical properties of a typical single junction isolator are:                insertion loss: ˜0.25 dB        isolation: ˜20 dB        return loss: ˜20 dB        bandwidth: ˜3 to 5%.        
Therefore, single-junction circulators are often combined together sequentially, connected serially in a coplanar adjacent configuration either side-to-side or end-to-end to attain a higher overall isolation—the higher overall isolation resulting from the additive sum of the isolation provided by each single-junction circulator. Many microwave ferrite circulators with such a sequential configuration are known. FIG. 2a (PRIOR ART) shows a schematic of two cascaded single junction circulators in a sequential coplanar configuration used to construct a dual or double junction circulator/isolator.
Conventionally, as shown in FIG. 2b (PRIOR ART), a double junction isolator 20 includes a body housing 42 having two communicating cylindrical cavities 44 arranged adjacent to each other for holding their individual stripline conductor circuits 46 serially connected as one in an abreast coplanar configuration, ferrite elements 14 that are magnetically biased by the magnets 16, and an electrical ground plane provided by pole pieces 18 in a stacked assembly. As can be seen from FIG. 2b (PRIOR ART), such a double junction circulator/isolator is at least double the physical size of a single junction circulator/isolator and adds to the complexity of the construction of the body housing needed to accommodate two cylindrical cavities in a side-by-side configuration. Furthermore, each ferrite element 14 associated with the stripline conductor circuit 46 in each cylindrical cavity 44 requires a separate magnet 16 to provide the required bias for the operation of the double junction circulator. For this reason, in such a conventional double junction circulator/isolator 20 as shown in FIG. 2b (PRIOR ART), a separate and individual magnetic biasing system has to be established in each cylindrical cavity to provide the required bias to the respective ferrite element. For such a double junction microwave ferrite circulator, the isolation provided by each single junction will still be typically ˜20 dB, but the input and output ports of the circulator/isolator will now be protected by an isolation level of at least 40 dB in the case of reverse signals.
Depending on the isolation required, two, three or more single-junction circulators may be sequentially connected together. However, increasing the number of sequentially-connected circulators generally increases the physical footprint size, the number of components, the total weight and the complexity of the assembly of such a microwave ferrite circulator device, and hence substantially increases the cost of the circulator device. Presently, a typical double junction circulator/isolator device costs at least twice as much as a single junction circulator/isolator. Moreover, while single junction isolators are sequentially connected together to improve isolation performance, the drawback of such a connection is the increase in insertion loss of such a ferrite device. For example, a typical insertion loss for a sequential double junction circulator/isolator is typically ˜0.5 dB as compared to 0.25 dB for a single junction circulator/isolator, which consequently reduces the level of the forward signal of microwave energy routed by this circulator/isolator by a factor of 12%, decreasing overall power handling capability and output efficiency.
Apart from the insertion loss incurred in the path of the signal within the stripline conductor circuit in a circulator/isolator, the other known major loss contributor is the conductor loss in the cylindrical cavity structure. In order to achieve high isolation performance by serially connecting stripline conductor circuits as in FIG. 2a (PRIOR ART) for the construction of a conventional double junction circulator/isolator device 20, multiple cylindrical cavities 44 have to be employed to house the stacked assembly of components (stripline conductor circuits, ferrite elements, magnets, electrical ground planes) needed for the operation of the double junction circulator, thus contributing significantly towards the increase in overall insertion loss.
In general, the physical footprint and volume is proportional to the cost of the required housing. In conventional well-known microwave ferrite junction circulator/isolator devices, the physical footprint contributes anywhere from 20% to upwards of 50% of the total cost. Hence, high isolation performance should be achieved within the smallest feasible physical footprint and volume.
U.S. Pat. No. 5,347,241 (PANARETOS et al) describes a dual-junction circulator based on back-to-back four-port microstrips. A microstrip includes a conducting strip separated from a single ground plane by a dielectric substrate. The device of PANARETOS includes two single junction microstrip circulators whose substrates do not lie in coplanar fashion but in a back-to-back fashion interconnected with a coaxial feedthrough that runs through both substrates and ground plane. Although the back-to-back configuration may be used to reduce the physical footprint area, the physical footprint is still greater than that of conventional single junction stripline circulators. Furthermore, the use of coaxial interconnects increases the overall volume (size) of the dual-junction circulator. The back-to-back configuration of the microstrip circulators also results in several limitations in the device of PANARETOS. Firstly, it can only be used for a dual-junction unit, and cannot be extended to a multi-junction device. Secondly, the magnetic system in the device of PANARETOS is relatively complex to provide the magnetic bias required for non-reciprocal operation of the dual-junction microstrip circulator. In one embodiment, as shown in FIG. 3a (PRIOR ART), PANARETOS uses a common magnet 16 for both the circulators, but such a mutual magnetic biasing system can only magnetize the ferrite element 14 of each circulator in opposite directions for the operation (i.e. when one microstrip circulator has counter-clockwise circulation, the other microstrip circulator will only circulate clockwise). In another embodiment as shown in FIG. 3b (PRIOR ART), two magnets 16 are used to magnetize the ferrite element 14 of each circulator which require same magnetic orientation for the operation. Magnetic shielding 50 is therefore required to reduce the interaction between the magnets 16 of the two separate magnetic biasing systems, henceforth limiting the operation of such a circulator to below ferrimagnetic resonance region. Finally, in both embodiments of U.S. Pat. No. 5,347,241 the magnetic return path from the magnetized ferrite element 14 of each circulator cannot provide a homogenous magnetic field for its operation, which limits the electrical performance and ferrimagnetic region of operation.
Accordingly, there is a need for a microwave ferrite circulator/isolator with high isolation performance, low insertion loss and a compact physical configuration.